The aviation industry relies upon numerous navigation aids in order safely to take off, navigate enroute, and land aircraft. Such navigation aids (naviads) include, for example, the instrument landing system (ILS), very high frequency omni-directional range (VOR) system, and the like. GPS, however, is increasingly being accepted as an alternative to traditional navigation aids, including even ILS.
Essentially, GPS is a space-based radio positioning network for providing users, equipped with suitable receivers, highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous orbits around the earth.
FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of "on orbit" spare satellites (not shown) for redundancy. The GPS satellites 101 are located in orbital planes, having an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles), and typically complete an orbit in approximately 12 hours. This positions each of the GPS satellites 101 in such a manner that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
GPS position determination is based upon a concept referred to as time of arrival (TAO) ranging. The orbiting GPS satellites 101 each broadcasts spread-spectrum microwave signals encoded with positioning data. The signals are broadcast on two frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, with the satellite ephemeris (positioning data in an earth centered, earth fixed, coordinate system) modulated using bi-phase shift keying techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise time of transmission. A user receives the signals with a GPS receiver. The GPS receiver is designed to time the signals and to demodulate the satellite orbital data contained in the signals. Using the orbital data, the GPS receiver determines the time between transmission by the satellite and reception by the receiver. Multiplying this time by the speed of light gives what is termed as the pseudo-range measurement of that satellite. If the GPS receiver clock were perfect, this measurement would be the range measurement for that satellite, but the imperfection of the clock causes the two measurements to differ by the time offset between actual time and receiver time. Thus, the measurement is called a pseudo-range, rather than a range. However, the time offset is common to the pseudo-range measurements of all the satellites. By determining the pseudo-ranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well as the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT with great accuracy, and use this information to navigate safely and accurately from point to point, among other uses.
While this position is more accurate than that obtainable using conventional navaids, there are aviation applications (e.g., take off and landing phases of flight) where an even greater level of accuracy is required. To attain these levels of accuracy, DGPS technology is employed.
DGPS functions by observing the difference between pseudo-range measurements determined from the received GPS signals with the actual range as determined from the known reference station point. The DGPS reference station determines systematic range corrections for all the satellites in view based upon the observed differences. The systematic corrections are subsequently broadcast to interested users having appropriate GPS receivers, and thereby enable the users to increase the accuracy of their GPS determined position. DGPS service and the supporting industry (e.g., avionics manufacturers) are increasingly being employed throughout the world.
In addition to accuracy, two other qualities are particularly important in the avionics field. The first of these qualities is reliability. Reliability in this context refers to the probability that a given avionics system will remain operational (e.g., continuity of service). The second of these qualities is integrity. In this context, integrity refers to the capability of equipment to assure that hazardously misleading information (HMI) is not generated and output from the equipment. Aircraft landing systems require high integrity levels in order to assure that a high enough level of safety is achieved. This integrity is usually expressed negatively, i.e., in terms of the probability of generating HMI during an hour of operation, or during an approach. To meet what is termed a "flight-essential" level of safety means to meet the requirement that the probability of generating HMI must be less than approximately 1 part in 10.sup.7 during an approach for passenger-carrying aircraft.
It is generally accepted practice that single-thread equipment (e.g., equipment without redundancy, in particular equipment that involves software and firmware) is considered to achieve only about 1 part in 10.sup.5 per approach. To achieve the 10.sup.7 level of integrity or better, redundant GPS receivers are usually used. This makes the total cost of the GPS avionics system quite high.
For example, large commercial air carriers (e.g., a commercial airline) typically employ redundant systems to achieve both the required degree of reliability (or continuity of service) and integrity. Two or more GPS receivers are installed into a GPS system of an aircraft. Each system is capable of functioning independently of the other, such that a failure in one system does not affect the operation of the other redundant system. Hence, the GPS system remains operational (e.g., GPS navigation information is still available) even though one GPS receiver has failed. This increases the reliability of GPS system, since for a system failure to occur, more than one GPS receiver must fail. Thus, even though, one GPS receiver has failed, the aircraft can continue in service until a convenient time of repair occurs.
In addition to increasing reliability, the incorporation of two or more GPS receivers into a GPS system increases integrity. For example, where two or more GPS receivers are installed, each of the receivers monitors the GPS signals and the differential corrections independently. This allows the GPS receivers to check the "health" of each other (and achieve the required 10.sup.7 level of integrity). If the GPS receivers do not agree, if their respective solutions differ by more than some pre-determined amount, the GPS system declares a fault, thus warning the rest of the avionics systems in the aircraft (e.g., the flight director) that the GPS system is currently unusable, and warning the air crew to utilize other navigation instruments or take other appropriate action (e.g., abort the approach).
While reliability is very important to some users, integrity is very important to all users. For example, with commercial air carriers, where flight cancellations due to maintenance are prohibitively costly, reliability is very important. Hence, very expensive and often redundant GPS receiver systems are installed. However, with a corporate aircraft or even a general aviation aircraft, the down time for a repair is much less costly, and hence reliability is less important. But in order to achieve satisfactory levels of integrity, the same very expensive, often redundant systems must be installed.
In particular, as the Wide-Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS) is phased in, costs of redundancy become prohibitive for many potential users. The problem is exacerbated by the fact that the current ILS landing system allows for single-thread operation for many users. While commercial air carriers are not cost sensitive and generally have redundant systems installed for reliability, many other potential users are precluded from installing and using GPS systems having the required degree of integrity because of cost.
Thus, what is required is a GPS system which provides a flight-essential degree of integrity. What is required is a system which supports the use of inexpensive GPS receivers, yet still meets the flight-essential integrity requirements. What is further required is a system which supports other navigation applications in addition to aviation. The present invention provides a novel solution to the above requirements.